The present invention relates to a method and apparatus for analyzing a damped structural specimen to determine with certainty significant parameters defining the characteristics of the specimen. More particularly, the present invention relates to method and apparatus for determining undamped mobility values and modal accelerations of the damped specimen, such as an airframe, for further use in identifying and improving the dynamic characteristics of the specimen.
Structural dynamic testing carried out in the early developmental stages of a new aircraft is one of the most expensive and time consuming tasks that confronts design engineers. Engineers must obtain directly from test data on an actual or scale model of the aircraft the information needed to predict performance characteristics and eliminate many dynamic problems that cannot be anticipated before the new aircraft is completely reduced to its first physical form. Obtaining meaningful information regarding the significant characteristics of an aircraft and reducing such information is known as system identification. System identification is used to confirm theoretical analysis of the aircraft characteristics and serves to assist the engineer in decision-making processes leading to an improved and more reliable product.
Methods of testing and test data analysis for improving the reliability and performance of aircraft are needed by both industry and governmental agencies, and the more accurate the analysis and identification of significant parameters of the aircraft system, the greater will be the success in achieving improved reliability and performance. The most common dynamic test carried out on new airframes is a vibration or shake test in which vibratory forces are applied to significant points of the airframe, for example, the mounting points for the lifting rotor of a helicopter, and measurements are taken at the same and other significant points in areas such as the cockpit, engine compartments, the autopilot compartment and the empennage to determine the impedance and transmissability characteristics of the airframe. The most common parameter measured at any point on the airframe is acceleration since this parameter is closely associated with forces, and accelerometers are readily available instruments.